Cardiac arrhythmias such as tachyrhythmia frequently result in ventricular fibrillation which in turn often results in death. Ventricular fibrillation is effectively treated by defibrillation, the non-synchronized delivery of electrical energy to the heart which stops the ventricular fibrillator and allows the heart to resume a normal rhythm.
The electrical energy is commonly delivered to the heart by two methods. Transvenous cardioversion leads are introduced to the heart through a catheter inserted through the venous system. This method is most appropriate to allow defibrillation when it is anticipated that defibrillation may only be required for a temporary period. It has the advantage that the energy source necessary to provide the electrical stimulus may remain outside the body.
The second method involves the use of patch electrodes that are permanently implanted on the surface of the heart. The energy source is permanently implanted as well. The disadvantages of the second method are that complex surgery is required and that the energy source is bulky, causing a substantial protrusion from below the surface of the skin. This method allows for immediate defibrillation any time the implanted system senses a ventricular defibrillation. It has been demonstrated to be an effective solution that has been widely employed for many years. It is intended to be a permanent solution, requiring only that batteries in the implanted energy source be replaced at relatively long intervals such as about three years. Finally, this method is entirely portable, allowing full mobility of the patient having such an implanted system. It is commonly believed that the benefits of the implanted systems far outweigh their disadvantages.
The implanted patch electrodes that transmit the electrical energy to the surface of the heart have various functional requirements that are difficult to achieve. These include biocompatibility, flexibility, effective attachment methods and the necessity to distribute the electrical charge so that it may be effectively delivered without burning or otherwise damaging the tissue to which it is attached.
Many various patch electrode designs have been proposed. U.S. Pat. No. 4,291,707 discloses a defibrillator electrode comprising a tissue contacting layer of electrically conductive wire mesh, the mesh having a backing layer of silicone rubber. The silicone rubber also encloses the periphery of the wire mesh. The backing layer of silicone rubber may optionally incorporate a reinforcing layer of Dacron.RTM. fabric.
U.S. Pat. No. 4,542,752 describes an implantable electrode tip assembly wherein the conductive surface comprises a layer of porous carbon deposited by glow discharge plasma onto the outer surface of a porous, electrically conductive substrate such as porous titanium.
U.S. Pat. 4,972,846 teaches the construction of a patch electrode comprising an insulating layer of porous expanded PTFE laminarly bonded to an electrically conductive layer of metal plated porous expanded PTFE.
U.S. Pat. No. 5,087,242 describes a hydratable skin-contact bioelectrode comprising a layer of conductive, hydratable material, a layer of conductive sheet material, and a support base on which the other layers are mounted. The layer of conductive, hydratable material is capable of absorbing solutions but not capable of discharging the absorbed solution by mechanical means such as squeezing in the fashion of sponges or other fibrous masses. The conductive, hydratable material may be a fibrous polymer or other type of matrix material impregnated with a hydratable polymer, or a layer of granulated polymer between two layers of hydrophilic material. Materials described as useful for the conductive electrode surface include polyethylene oxide, polyacrylamide and ammonium polyacrylate.
Patent GB 2,182,566 A relates a patch defibrillator electrode comprising a layer of electrically conductive metal foil having a pattern of slits through the entire thickness of the foil to provide the foil with good flexibility. The foil layer has an optional insulating backing of a porous insulating material such as Dacron fabric. The porous insulating material may also be used to enclose the peripheral edges of the foil. The porous insulating material may optionally incorporate a biologically active agent for inhibiting thrombus formation.
U.S. patent application Ser. No. 07/456,113, now U.S. Pat. No. 5,148,806, teaches the construction of an electrode for use on a living body, t electrode comprising porous polytetrafluoroethylene containing a conductive powder, preferably either carbon black or metal.